For the purpose of power transmission, gear devices and pressure oil devices have been widely used. There is also known a method involving a traction drive which comprises transmitting power via an oil film between steel rotating members. Traction drives have also been used in industrial machinery because of the advantages that they generate little vibration and noise during operation (due to the absence of interlocking gears) and that they permit a continuously variable transmission. A study is underway to adopt traction drives to automobiles and agricultural tractors because traction drives provide energy transmission which results in energy saving.
In traction drives, the selection of a lubricating oil is very important because power is transmitted via an oil film present in the contact area between rotating members. Since power is transmitted by shearing of the oil film which becomes very viscous due to the high pressure at the contact area, it is preferred that the lubricating oil used in traction drives has a high shear resistance to obtain a high power transmitting performance.
As a measure of power transmitting performance, generally use is made of the traction coefficient which is the ratio of the tangential force to the perpendicular load. Also, low viscosity is preferred in order to minimize losses in power transmission due to resistance to agitation.
When traction drives are used in areas where the heat load is high, such as in the transmission of an automobile, the oil temperature rises to as high as 100.degree. C. or more. The problem is then encountered of a decrease in the traction coefficient due to the increased temperature. It is important to minimize the decrease in the traction coefficient following such an increase in temperature.
The preferred lubricating oils for traction drives are naphthenic hydrocarbons and many are disclosed e.g., in JP-B-46-338, JP-B-46-339, JP-B-47-35763, JP B-48-42067, JP-B-48-42068, JP-B-61-15918, JP-B-61-15919 and JP-B-61-15920 (the term "JP-B" as used herein means an "examined Japanese patent publication").
The disclosed lubricating oils, however, are not fully satisfactory in performance because many of them have shortcomings, e.g., even if a high traction coefficient is exhibited at near room temperature, it decreases as the temperature rises or its efficiency is lowered due to high viscosity.